Resonator structure



March 26, 1957 A. M. GUREWITSCH RESONATOR STRUCTURE Filed Dec. 19, 1952 l I I Inventor: Anatole M.Gur"ewitsch,

His Attorney- RESONATOR STRUCTURE Anatole M. Gurewitsch, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application December 19, 1952, Serial No. 326,818

7 Claims. (Cl. 333-82) My present invention relates to high frequency resonators and, more particularly, to resonators finding useful application in apparatus for imparting high energy to charged particles.

My invention is applicable in connection with apparatus of the type disclosed and claimed in United States Patent No. 2,485,409, patented October 18, 1949 by Herbert C. Pollock and Willem F. Westendorp and assigned to the assignee of the present invention. Such apparatus comprises means for initially accelerating charged particles in an orbital path by the action of a time-varying magnetic field and for thereafter producing continued acceleration by a localized electric field of cyclically varying character.

In one form of this apparatus, the device employed for producing the localized electric field comprises a space resonant structure suitably located along the orbital path of the charged particles within the region of the time varying magnetic field. This disposition of the resonant structure necessitates the provision of an arrangement for preventing excessive eddy currents from flowing in the structure; because the high intensity magnetic field traversing the structure would otherwise generate eddy currents which cause undesirable distortions in the magnetic field and also result in serious overheating of the structure. As is disclosed in my United States Patent No. 2,553,312, issued May 15, 1951 and assigned to the assignee of my present invention, the conductors of the space resonant structure so employed in charged particle accelerators can be longitudinally subdivided to reduce the flow of eddy currents therein. Compensation for eddy currents which still tend to flow can be obtained by the interconnection of the longitudinally extending strips in the manner disclosed in my United States Patent No. 2,579,315, issued December 18, 1951, and also assigned to the assignee of my present invention.

it is pointed out in my aforesaid United States Patent No. 2,579,315 that it is further advantageous to inter connect the longitudinally extending strips of the resonant structure or resonator because, if the strips are not connected to each other, the structure presents electrically a plurality of coupled conductive members which are sufiiciently loosely coupled at high frequency to have a great number of undesirable modes of oscillation. in some applications, I have found it highly desirable to be able to accomplish such interconnection of the longitudinally extending strips for high frequency currents at various positions along the resonator without providing closed paths for the flow of eddy currents. A principal object of my present invention is, therefore, to provide a means of interconnecting the longitudinally extending strips of a resonator of the type described wherein the interconnections can be made for high frequency currents without providing closed paths for the flow of eddy currents and a further object is to localize the fringing electric fields which occur at the ends of the resonator.

According to one aspect of my invention, I provide a resonator which comprises an inner conductor that innited States Patent cludes a plurality of separate conductive strips extending longitudinally along the inner surface of a tubular section of dielectric material and an outer conductor that includes a plurality of separate conductive strips extending longitudinally along the outer surface of the tubular section. The longitudinally extending strips of the inner and outer conductors can each be coupled together for high frequency currents by at least one peripherally extending conductive loop which is capacitively coupled to the respective conductor, whereby the strips of each conductor are coupled together for high frequency currents but are separate in so far as the flow of eddy currents is concerned.

The features of my invention desired to be protected herein are pointed out with particularity in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawing in which Fig. 1 is a sectionalized view of a resonator structure suitably embodying my invention and Fig. 2 is a cross section taken envelope 2 of dielectric material that defines within its interior an annular evacuated chamber. Envelope 2 provides, in connection with charged particle accelerating apparatus such as that disclosed in the above-mentioned Pollock and Westendorp Patent No. 2,485,409, a generally circular orbital path indicated by broken line 3, along which charged particles, e. g., electrons derived from ;a suitably energized source, can be accelerated to high energy levels. A high resistivity coating 2, of a material such as stannic chloride deposited upon the interior surface of envelope 2, reduces the effect of wall charging by stray charged particles. Resonator 1 cornprises an outer conductor 4 and an inner conductor 5 which are respectively supported upon the outer and inner surfaces of a section 6 of envelope 2. In order to obtain a vacuum-tight annular chamber within the interior of envelope 2, section 6 can be suitably sealed to the remainder of envelope 2 by gla-ss-to-glass seals indicated at 7 and 8. During operation of the accelerator apparatus, resonator 1 is traversed by a high intensity, timevarying magnetic field which has a direction approximately perpendicular to the plane of the paper; hence, outer conductor 4 and inner conductor 5 are both longitudinally subdivided as shown to form pluralities of longitudinally extending strips 9 and 10 for the purpose of reducing the flow of eddy currents in the conductors.

One convenient way of forming conductors 4 and 5 is as follows. Layers of silver paint are applied to the surfaces. The thickness of these layers can be increased by means of electroplating, or by repeated painting and baking, for the purpose of obtaining layers having thicknesses greater than the depth of penetration of the high frequency current which flows therein when resonator 1 is excited. As an order of magnitude, the layers may have a final thickness of about 1 to 1 /2 mils when the resonator is to be excited at me. Of course, the layers should not have a thickness any greater than is necessary for the proper flow of high frequency current, because otherwise excessive eddy currents might be generated therein. Various portions can be removed from the con ductive layers to provide longitudinally extending conductive strips 9 and 10 by masking during the painting operation, or by burning in grooves (after baking and electroplating) with a tungsten disk which is rolled along the conductive surfaces while a relatively large current Patented Mar. 26, 1957 is p assed through the contact area. Alternatively, longitudinally extending strips 9 and 10 can consist of a suitable conductive material such as copper and can be attached tosection 6 with asuitable adhesive-material, e. g, an alkyd resin prepared by reacting a polybasic acid and polyhydric alcohol, such as a resin being prepared from glycerol and phthalic anhydride.

It .will nowbe understood that the length of inner conductor may be so selected in connection with the frequency of excitation of resonator 1 that the resonator will operate as a half-wave, open-circuited, concentric line resonator. High frequency energy can be supplied through a concentric transmission line (not shown) which is connected to resonator 1 by means of a. concentric line connector llyhaving an inner conductor 12' and an outer conductor 13 directly connected respectively to inner conductor 5 and outer conductor 4. In order to obtain proper excitation of resonator l inthis manner, connector 11 must be displaced somewhat axially from the center of the resonator. When resonator 1 is excited, the space between outer conductor 4 and inner conductor 5 constitutes in effect a space resonant system comprising a halfwave transmission line section. Accordingly, if resonator 1 is excited at the proper frequency, cyclically reversible electric fields may be made to appear at both ends of the resonator structure and to extend inwardly to act upon charged particles moving along or in the vicinity of orbital path 3. These high frequency fields fringing from the ends of resonator 1 are a sine function of time and are effectively localized near the ends of the structure, thereby reducing high frequency energy losses, by the portions 14 of outer conductor 4, which extend beyond the respective ends of inner conductor 5, and constitute wave guides operating below cutoff frequency. The resistivity of coating 2' is made sufliciently high to prevent portions of the coating from forming concentric lines with portions 14.

From the foregoing, it will be apparent that, by choosing the frequency of the reversal of the high frequency electric fields provided by resonator 1 to correspond to the frequency of rotation of charged particles moving within envelope 2 along orbital path 3, an increase in the energy level of such charged particles can be effected in accordance with the principles outlined in the aforesaid Pollock and Westendorp Patent No. 2,485,409. It will, however, appear to those acquainted with this art that resonator 1 must be foreshortened so that its electrical half-wave length does not constitute physically a halfwave length of the circulating charged particles. If such were the case, "the net energy gain imparted to the charged particles by the high frequency fields at the ends of resonator 1 would be zero. Therefore, the dielectric material of section 6 must be chosen to provide a dielectric constant which is in excess of 1. A material having a dielectric constant suitable'for this purpose can be aglass selected from the glasses described by Louis Navias and R. L. Green in the Journal of The American Ceramic Society, vol. 29, No. 10, pp. 267-276 (1946). Glass No. 5 Whose composition is set forth on p. 270 of this article (69% SiOz, 28% B203, 1% A1203, and 2% LizO) has, for example, been found to be quite satisfactory.

As has been mentioned heretofore, the separate longitudinally extending conductive strips 9 and 10 of outer and inner conductors 4 and 5', respectively, are sufficiently loosely coupled to cause resonator 1 to have a great number of undesirable modes of operation. To increase the high frequency COupling between the various strips of each conductor, the strips can be made longitudinally discontinuous to provide a peripherally extending conductive loop such as loop which directly interconnects the strips 10. Alternatively, the strips of each conductor can be coupled-together for high frequency currents by a peripherally conductive loop such as loop 16 which is attached to the exposed surfaces of strips 9. Quite apparently, it-is advantageous to limit the axial or longitudinal extent of the loops in order that excessive eddy currents will not be generated therein.

While directly connected loops 15 and 16 respectively provide some degree of improvement in the high frequency coupling between the strips 9 and 10, entirely satisfactory operation of resonator 1 very often demands further increase of the high frequency coupling. It is not possible, however, to place more than one directly connected peripheral conductive loop upon each conductor since, if a second directly connected loop is added to one of the conductors, longitudinally and laterally extending loops enclosing a substantial portion of the time-varying field will result, whereupon large eddy currents will flow in the paths thus provided. In order that substantially increased high frequency coupling can be obtained without any attendant increase in the flow of eddy currents, there are provided peripherally extending conductive loops 17 and 18 which are capacitively coupled respectively to the outer conductor 4 and inner conductor 5 of resonator 1. Conductive loops 17 and 18 preferably comprise thin layers of a material such as silver, which are deposited respectively upon peripherally extending loops 19 and 20 that can consist of a dielectric material such as mica or polystyrene. It has been found that by proper selection of the thicknesses of the dielectric loops 19 and 20, a very substantial improvement in the high frequency coupling among the various strips of. the respective conductors of resonator 1 can be obtained without appreciably increasing the magnitude of eddy currents generated in the resonator structure by the time-varying magnetic field of associated accelerator apparatus. Although only one capacitively coupled, peripherally extending, conductive loop has been shownupon each of the outer and inner conductors of resonator 1, it is obvious that a plurality of such loops positioned at desired'axial positions may be employed in connection with each conductor if further increased high frequency coupling is desired. Conductive loops 17 and 18 preferably are narrower than dielectric loop s 19 and 20, respectively, in order that arcing between the loops and the resonator conductors will not occur.

While the invention has been described by reference to particular embodiments thereof, it will be understood that numerous. changes can be made by those skilled in the art without actually departing from the invention. For example, it is apparent that the capacitively coupled conductive loops of my invention can be utilized with other types of resonators having longitudinally subdivided conductors, and the advantageous employment of such loops is not restricted to the specific half-wave resonator illustrated. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In apparatus for the acceleration of charged particles along a path which is enclosed by an envelope of dielectric material, a resonator forming a section of the envelope comprising an inner conductor which includes a plurality of longitudinally extending conductive strips supported upon the inner surface of said section, an outer conductor which includes a plurality of longitudinally extending conductive strips supported upon the outer surface of said section, and means for coupling together for high frequency currents said longitudinally extending conductive strips of at least one of said conductors including at least one peripherally extending conductive loop capacitively coupled to said conductive strips of at least one of said conductors.

2. A resonator as in claim 1 in which said at least one peripherally extending conductive loop comprises a peripherally extending strip of dielectric material and a strip of conductive material attached to said strip of dielectric material.

3. A resonator comprising an inner conductor which includes a plurality of separate conductive strips extending longitudinally along the inner surface of a tubular section of dielectric material, an outer conductor which includes a plurality of separate conductive strips extending longitudinally along the outer surface of said section, and means for coupling together for high frequency currents said separate longitudinally extending conductive strips of at least one of said conductors including at least one peripherally extending conductive loop capacitively coupled to said conductive strips of at least one of said conductors.

4. A resonator as in claim 3 in which said at least one capacitively coupled peripherally extending conductive loop comprises a peripherally extending strip of dielectric material and a strip of conductive material attached to .said strip of dielectric material.

5. A resonator as in claim 3 in which said at least one peripherally extending conductive loop comprises a pcripherally extending strip of dielectric material and a strip of conductive material attached to said strip of dielectric material, said strip of conductive material having a lesser longitudinal extent than said strip of dielectric material.

6. A resonator comprising an inner conductor which includes a plurality of separate conductive strips extending longitudinally along the inner surface of a tubular section of dielectric material, said strips being conductively coupled together by a peripheral conductive loop connected directly to said strips and capacitively coupled together by at least one peripheral conductive loop capacitively coupled to said strips adjacent said directly connected loop, and an outer conductor which includes a plurality of separate conductive strips extending longitu- 6 dinally along the outer surface of said section, said lastmentioned strips being conductively coupled together by a peripheral conductive loop connected directly to said last-mentioned strips and capacitively coupled together by at least one peripheral conductive loop capacitively coupled to said lastnnentioned strips adjacent said last-mentioned directly connected loop.

7. In an apparatus for the acceleration of charged particles along a path which is enclosed by an envelope of dielectric material, a resonator forming a section of the envelope comprising an inner conductor which includes a. plurality of longitudinally extending conductive strips supported upon the inner surface of said section, an outer conductor which includes a plurality of longitudinally extending conductive strips supported upon the outer surface of said section and extending beyond at least one end of said inner conductor to act as a waveguide operating below cut-off frequency and to localize the electric fields occurring near at least one end of said resonator, and at least one peripherally extending conductive loop capacitively coupled to said conductive strips of at least one of said conductors.

References Cited in the file of this patent UNITED STATES PATENTS 2,485,409 Pollock et al. Oct. 18, 1949 2,512,468 Percival June 20, 1950 2,512,945 Kallmann June 27, 1950 2,553,312 Gurewitsch May 15, 1951 2,579,315 Gurewitsch Dec. 18, 1951 2,589,739 Shepherd Mar. 18, 1952 

